Electronic locks and keys are used in military applications to prevent unauthorized launching of missiles and arming of nuclear warheads, and in commercial applications to prevent unauthorized entrance into a given area or initiation of a process, for examples.
Mechanical locks, such as combination or lock and key types, are inadequate for military purposes since they can easily be opened, cannot be used where initiation or activation from a remote area is required, and can malfunction in extreme environmental conditions of temperature, etc.
Electronic locks and keys using shift registers have been used, but have not been entirely satisfactory from the standpoint of security and volume or space, for examples.
The present invention relates to a new and improved electronic lock, and lock and key combination, in which a large number of design options are available to make it statistically impossible to circumvent the lock with trial and error use of random codes, without the required key.
In accordance with the invention, an electronic key comprising first means for generating a first coded pulse train is connected to an electronic lock comprising: second means for generating a second coded pulse train; an EXCLUSIVE-OR gate having two inputs connected to the outputs of the two pulse train generating means to produce a logical ZERO output in response to the identical coded pulse trains or a logical ONE output in response to pulse trains that are different; an electronic gate having a first input connected to the output of the second pulse generating means; a binary output counter having a clock input connected to the output of the electronic gate, and means for producing an output at a selected count; and means, connected between the output of the EXCLUSIVE-OR gate and a second input to the electronic gate, for keeping the electronic gate open for transmission of the second coded pulse train to the output counter when the two coded pulse trains are identical, and for closing the electronic gate to stop the counter from counting when the two coded pulse trains are different.
In the embodiment disclosed as an example, the electronic gate is a first NAND gate, the means for opening and closing this NAND gate comprises: a second NAND gate having a first input connected to the output of the EXCLUSIVE-OR gate; and a toggle flip-flop having a toggle input connected to the output of the second NAND gate, an output connected to a second input to the second NAND gate, and means for initially producing a logical ONE output therefrom. In the example, the key includes a power supply and an oscillator or clock for generating a uniform pulse train, and each coded pulse train generating means comprises an N-stage binary counter connected to the clock and to an N-stage multiplexer having 2.sup.N control lines to which a given binary code is applied, where N is an integer greater than 1. The two coded pulse train generating means are as nearly identical as possible, to permit "opening" of the lock and production of the output signal, without closure of the NAND gates.
If any other key, having a different pulse frequency or pulse shape, is connected to the lock, the EXCLUSIVE-OR gate output changes to a logical ONE, which toggles the flip-flop to a ZERO output, which closes the NAND gates and prevents transmission of the pulses to the output counter.
Preferably, the clock for generating the uniform pulse train and the power source are parts of the key, to make it more difficult to open the lock. The flip-flop may be either a single flip-flop or the last flip-flop of a binary counter.